DEFORMATION FEATURES OF GNEISS AND UHP ECLOGITE FROM DUCTILE SHEAR ZONE AND ITS RELATION WITH SEISMIC VELOCITY ANISOTROPY: EVIDENCES FROM CORE SAMPLES AT DEPTH OF 680 ~ 1200M OF CCSD

Abstract

Several ductile shear zones developed at depth of 680 - 1200m of CCSD. The main rock types in the shear zone include gneisses and UHP eclogites. The strain axes of deformed quartz in gneisses and stretch garnets and omphacites in eclogites are expressed as X > Y > Z ratios. Their flinn coefficients vary from 0.11 to 0.27, 0.22 to 0.23 and 0.23 to 0.24, respectively. In the gneiss sample the C - axis fabric of deformed quartz shows a great circle normal to the foliation and a maximum near Z. With the strengthening of mylonitization, the fabrics become a great circle near the foliation and a maximum near Y. P- and S-wave velocities were measured at room temperature and pressure. Calculated P- and S-wave velocity anisotropies of gneisses vary from 30.17% to 60.97% and 11.52% to 35.79% , while those of eclogite vary from 0.17% to 11.19% and 2.41% to 6.70%, respectively. Seismic velocity anisotropies are mainly caused by lattice preferred orientation (LPO), shape preferred orientation (SPO) of the major minerals, and oriented microcracks. P-wave velocity anisotropies of gneiss increase with strengthening mylonitization. LPO and SPO of biotite, quartz and omphacite are responsible for the velocity anisotropy of deformed rocks. P-wave velocity anisotropies of water-saturated gneiss are lower than those of dry gneiss. The strongly reflective zone beneath the Donghai drill site can be explained by the impedance contrasts between the different lithologies. Contacts between eclogite/retrograde eclogite and biotite-plagioclase gneiss may give rise to strong seismic reflections. In addition, lattice preferred orientation-related seismic anisotropy can increase reflectivity, thus the mylonitized gneiss in ductile shear zone may be good reflectors. The strong anisotropy of ductile shear zone provides important constraints on interpretations of seismic deep reflection results.